1. An investigation into turbine performance and wind flow modelling
under cold weather driven stable atmospheric conditions
Winterwind 2013 (Östersund, Sweden - 13 February 2013)

2. GL Garrad Hassan – Independent Renewable Experts
Almost 1000 staff, in 44 locations, across 26 countries
Heerenveen
Sint Maarten
Kaiser-Wilhelm-
Koog
Glasgow Copenhagen
London Hinnerup
Slough Oldenburg
Bristol Hamburg Beijing
Vancouver Cork Warsaw
Seoul
Paris Imola
Ottawa Tokyo
Portland Lisbon Izmir Shanghai
San Diego Barcelona Cairo Mumbai
Zaragoza
Montreal Madrid Bangkok
Peterborough Bangalore
Austin Singapore
Querétaro Newcastle
Porto Alegre Wellington
Santiago Cape Town Melbourne

3. Content
Cold climate driven stable atmospheric conditions in the Nordic Region:
1 - Consequences to the wind flow (challenges) and how it translates
2 - How complex CFD flow modelling can help with the wind flow modelling challenges
3 - Impact of high frequency of stable atmospheric conditions on turbine performance

4. 1 –2–3
Stable atmospheric conditions
What is it and how it translates

5. Radiative cooling driven stable atmospheric conditions
- Unstable, neutral, stable and very stable atmospheric conditions
- Actual
- Dry-adiabatic
- High frequency of stable atmospheric conditions

6. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

7. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

8. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

9. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

10. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

11. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

12. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

13. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

14. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

15. Radiative cooling driven stable atmospheric conditions
- When do stable and very stable atmospheric conditions happen in the Nordic region

16. High frequency of stable atmosphere and wind conditions
Low TI as a proxy for stable
atmospheric conditions

17. Wind Flow modelling challenges
Wind speed at mast A/Wind speed at mast B = Speedup
1.10
1.05
1.00
0 30 60 90 120 150 180 210 240 270 300 330
0.95
Speedup
0.90
0.85
0.80
0.75
0.70
0.65
Direction Sector (deg)
Stable Unstable Total

18. 1– 2 –3
Complex CFD flow modelling for
stable wind flow challenges

19. What is CFD (Computational Fluid Dynamics)
• Most accurate description of wind flow: Navier-Stokes Equations (below)
• Very difficult to solve!
• Linear models simplify this equation - some accuracy is sacrificed
• CFD makes fewer changes – less loss of accuracy:
• Reynolds-Average Navier-Stokes (RANS)
• Advanced software required
• Modern computing power required
• Like a wind tunnel on your computer

20. Wind Flow modelling under stable conditions
Directional speedup errors (95 mast pairs at 16 sites)
Neutral CFD
Linear
(WAsP)

21. Stable + Neutral CFD approach (GL GH approach)
Directional speedup errors (95 mast pairs at 16 sites)
Stable+Neutral Average Errors
error is lower
Linear (WAsP) = 9.6%
Neutral CFD = 9.5%
Stable + Neutral (CFD) = 6.6%

22. 1– 2 – 3
High frequency of stable conditions
and turbine performance

23. Performance of wind turbines under stable conditions in the
Nordic Region – What the theory suggests
P = 1/2ρ(u)3(1+3TI2)ACp
2000
1500
Power (KW)
0%
5%
1000
10%
15%
20%
500
0
2 4 6 8 10 12 14
Wind speed (m/s)

24. Performance of wind turbines under stable conditions in the
Nordic Region
Results for IEC Power Curve Measurements of 5 mast/turbine pairs in Sweden
106%
measured power curve production
104%
102%
Low TI / High TI
T1
100% T2 No partial icing cleaning
98% T3 and no temperature
96% T4 filtering
T5
94%
Average
92%
6 7 8
Annual mean wind speed (m/s)

25. Performance of wind turbines under stable conditions in the
Nordic Region
Results for IEC Power Curve Measurements of 5 mast/turbine pairs in Sweden
106%
measured power curve production
104%
102%
Low TI / High TI
T1
100% T2 Data cleaned for partial
98% T3 icing and filtered
96% T4 excluding temperatures
T5 below 0˚C
94%
Average
92%
6 7 8
Annual mean wind speed (m/s)

26. Performance of wind turbines under stable conditions in the
Nordic Region
Results for IEC Power Curve Measurements of 5 mast/turbine pairs in Sweden
106%
measured power curve production
104%
102%
Low TI / High TI
T1
100% T2 Data cleaned for partial
98% T3 icing and filtered
96% T4 excluding temperatures
T5 below 0˚C and different
94%
Average site calibration speedups
92%
applied for stable and
6 7 8
Annual mean wind speed (m/s)
unstable conditions

27. Performance of wind turbines under stable conditions in the
Nordic Region
Results for IEC Power Curve Measurements of 5 mast/turbine pairs in Sweden
Without different site calibration speedups With different site calibration speedups
110% 110%
105% 105%
measured power curve
measured power curve
100% 100%
Low TI / High TI
Low TI / High TI
95% 95%
90% 90%
85% 85%
80% 80%
4 5 6 7 8 9 10 11 12 13 14 15 16 17 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Wind speed (m/s) Wind speed (m/s)
T1 T2 T3 T4 T5 Average T1 T2 T3 T4 T5 Average

28. Conclusions
There is high frequency of stable and very stable atmospheric conditions in sites
across the Nordic Region caused by radiative cooling
This presents a challenge for both wind flow modelling and turbine performance
Stable CFD reduces the error in wind flow modelling at these sites
Theory shows that low turbulence reduces turbine performance in raising part of
the power curve
Preliminary Nordic data supports this, however shows recovery in the “knee” of
the power curve
Need to gather more Power Curve Measurement data across the Nordic region
Careful consideration should be given to winter Power Curve Measurement data,
as partial icing affects more low TI data than high TI, and the anemometry used
Site calibration speedups for stable and unstable conditions should be considered
separately as suggested by the new draft of the IEC standard

29. Thank you
Any questions?
Contact:
Carla Ribeiro, Senior Team Leader Nordic Region
carla.ribeiro@gl-garradhassan.com
With Thanks to:
Simon Cox
Bob Hodgetts
Andrew Tindal
GL GH CFD Team
GL GH AMOS Team